Every interaction you have with a character raises the relationship by one point. If you're having trouble keeping up a certain character's relationship points, spend more time with them in addition to giving them gifts. You don't really need to raise the relationship points for any particular reason, only maintain them; it's the love/corruption points that you want to raise in order to unlock more outfits and content.
Edit: Obviously, you want to keep the relationship points above 5 if you don't want to get NTR'd. But yeah, it's not like 70 relationship points really affects anything compared to 50 relationship points.
But talking to them dosn't effect relationship, looked before and after each talk, no change.
So giving flowers, chocolate, talking all you can and relationsship are still dropping... then it is impossible to keep above 5.
Ahh only one type of talking increases relationship...
That makes no sence, so to keep a steady relationship, you have to keep giving presents and talk. But then you are locked out of increasing love/corruption dialogues forever...
Sorry thats just bad gameplay mechanics and is wierd, why would relationship drop if you are not showering with presents and dialogue?
that interaction in some way is needed, ok fine, thats makes sence, but this... no.